Current responsive instrument



Aug. 27, 1940. H. F. GRAVE yCURRENT RESPONSIVE INSTRUMENT Filed Dec. s,1957 L 500 FREQUENCY l/V CYCLES PER SECO/VD HA na af :fm/fs Hfs/sumas/VD IPED/INCE 0F CUR/:TENT RESPONSIVE amr ,4r a :ya is Pm seco/va ma az043 als 25'00 dan 35'00 40'00 FREQUENCY /lv cycL :s PER smo/vn /sbainventor:

Hans E Grave, b9

Hi Attoreg.

Patented Aug. 27, 1940 UNITED STATES PATENT OFFICE- CURRENT RESPONSIVEINSTRUMENT New York n"Application necember 8, 1937, serial No, 178,820

' In Germany December 14, 1936 My invention relates to currentresponsive instruments and concerns particularly both current andvoltage-measuring devices.

It is an object of my invention to providean g improved arrangementhaving high sensitivity for measuring alternating currents and voltages.

It is also an object of my invention to overcome frequency error.

It is still a further object of my invention to i@ provide anarrangement which is suitable for the use of current responsive elementshaving relatively high inductance. Further and other objects andadvantages will become apparent as the description proceeds.

In carrying outvmy invention in its preferred form I utilize 'a directcurrent galvanometer of the DArsonval type, for example, in the Graetzor Bridge connection of rectifiers and I provide a capacitativeimpedance in parallel with the bridge for the purpose of compensatingfor frequency errors.

My invention will be understood more readily from the following detaileddescription when considered in connection with the accompanying 25drawing and those features which are believed to be novel and patentablewill be pointed out in the claims appended hereto. In the drawing,

Figure 1 is a graph illustrating the principle of operation of myinvention when used in am- 8i) meters and in certain classes ofvoltmeters. Figure 2 is a diagram representingl the equivalent circuitof the conventional rectifier instrument connected in the Graetzcircuit. Figure 3 is a circuit diagram of one embodiment of myinvention. Figure 4 is a circuit diagram of another embodiment of myinvention Iutilizing a push-pull bridge circuit instead of thefour-rectier or Graetz bridge circuit. Figure 5 is a diagramrepresenting the equivalent circuit of the conventional rectifiervoltmeter in the Graetz circuit. Figure 6 is a circuit diagram of anembodiment of my invention arrangedfas a voltmeter and Figure 7 is agraph illustrating the principle of operation of my invention involtmeters. Like reference characteristics are utilized throughout thedrawing to designate like parts.

In view of the fact that the effect of dry rectifiers is dependent uponfrequency, instruments of the rectifier type equipped with copper-oxideor selenium rectifiers give indications which vary considerably withvariations in frequency. 'I'his phenomenon appears to result in partfrom the capacitance of the rectifier plates which causes a rectifyingeffect decreasing with increasing frequency, that is, a negativefrequency error` Inasmuch as the rectifier is intended to block currententirely in one direction any effect such as a capacitative effect whichpermits the current to' ow in either direction tends to make theinstrument read too low, that is, gives it a negative error.

However, when the galvanometer or instrument element is one having aconsiderable inductance the capacitative effect of the dry rectierstends to give the combined instrument a, positive error instead of anegative one for the reason that the dry rectifiers acting as condenserstend to store up charges which are prevented from flowing through theinstrument element except when a cur-rent is flowing in a positivedirection for the reason that the inductance of the instrument elementtends to oppose stoppage of the iiow of current therethrough.

'I'his problem arises particularly in case of large switchboard andrecording instruments having deflecting elements with large numbers ofturns and high inductance. If such an instrument arranged in theconventional Graetz circuit with dry rectiers is connected as an ammetera frequency Acharacteristic results such as that shown by curve A ofFigure 1. It will be seen that a positive frequency error occurs.

In accordance with 'my invention the error indicated is diminished orovercome by use of an artificial circuit with rectifier measuringinstru'- ments containing highly inductive deecting instrument elements.In the case of ammeters the artificial circuit preferably takes the formof a capacitative impedance connected in parallel with the terminals ofthe usual current responsive unit. The capacitative impedance preferablyconsists of a condenser and ohmic resistance in series. In the form ofthe invention shown in Figure 3 there is a Greetz bridge II consistingof four dry rectifiers I2, I3, I4, and I5 arranged in 40 two pairs, eachof which is connected in series opposition to the end terminalsconnected to the input terminals I6 and I1 of the current responsiveunit. There is a current responsive instrument element such as theDArsonvai galvanometer I8, for example, connected in the cross circuitof the bridge between the common terminals I9 and 2li-of the two pairsof series opposed dry rectifers I2, I3 and I4, I5 respectively. For thepurpose of obtaining frequency compensation a capacitative impedanceconsisting of a condenser 2i and a resistor 22 in series, are connectedacross the input, terminals I6 and I'I in parallel cooperativerelationship to the bridge II.

The principle of operation of the embodiment of the inventionillustrated in Figure 3 may be understood more readily from aconsideration of the equivalent circuit of the conventional Graetzcircuit rectier instrument represented in Figure 2.

In Figure 2, r represents the pass resistance of the dry rectiers l2 andI5. C represents the back capacitance of the dry rectiers I3 and I4respectively, the blocking resistance and the series capacitance beingneglected. R represents the ohmic resistance and L the inductance of theWinding of the current responsive instrument element I8. I representsthe current to be measured and In represents the current owing in theinstrument element I8. By setting up the equations of current andvoltage according to Kirchhofs Laws for the electrical circuitrepresented in Figure 2 and solving for In, the following equationresults:

It will be understood that y' is \/-1 and w is 2 1r times the frequency.For the values of C, L, R usually encountered in practice K1 increaseswith increasing frequency Within the usual operating range. The curve Arepresents the percentage error in terms of the indication at afrequency of 50 cycles per second plotted against frequency.

The use of the capacitative impedance in parallel co-operativerelationship to the rectifier bridge overcomes the increase in value ofIn with the increase in frequency. The corresponding error with thecapacitative impedance connected in parallel relationship is shown bycurve B in Figure 1.

In Figure 3, I have illustrated the application of my invention to theconventional Graetz bridge circuit but it will be understood that myvinvention is not limited thereto, for example, in the case of atransformer bridge or push-pull circuit as illustrated in Figure 4 thecapacitative impedance consisting of the condenser 2| and the resistor22 is likewise connected in parallel cooperative relationship to therectier bridge. The arrangement of Figure 4 includes a currenttransformer 23 having a primary winding 24 and a secondary winding 25with an intermediate tap 25. A single pair of dry rectifers I4 and I5connected in series opposition is arranged with the end terminals 21 and28 connected across the secondary winding 25 of the transformer 23. Theinstrument element I8 is connected in a cross circuit between theintermediate points of the bridge in the usual manner, in this casebetween the intermediate point 26 of the transformer winding 25 and thecommon terminal 29 of the rectiiiers I4 and I5. It will be understood,of course, that the optimum values of capacity and resistance of thecircuit elements 2| and 22 will not necessarily be the same in thearrangements of Figures 3 and 4.

If the conventional Graetz circuit instrument is arranged as a voltmeterwith a resistance multiplier 29, an equivalent circuit results such asillustrated in Figure 5. In this case the symbols previously used havethe same significance as in connection with Figure 2. In addition theresistance of the multiplier 29 is represented by the symbol S and theapplied voltage to be measured is represented by the symbol V. Thecurrent flowing through the moving coil element I8 is then representedby equation:

If the series resistance S is made so large that v the second and thirdterms in the Adenominator become negligible with respect to the rst termthe equation for the voltmeter becomes the equation previously given forthe ammeter. That is, voltmeters with high resistance multipliers behavelike ammeters and the current responsive units in such voltmeters may becompensated in the same manner as described in connection with Figures 3and 4 namely, by connecting a capacitative impedance in paralleltherewith. A corresponding voltmeter arrangement consisting of thecurrent responsive unit of Figure 3 in series with the voltagemultiplier 29 is illustrated in Figure 6.

If the series resistance of the voltmeter is made equal to zero insteadof being Very large the current in the current indicating coil I8 isgiven by the following equation:

JTC"r (RH-mL) (Wely-ai JwC jwC' In contra-distinction to K1,

decreases with frequency under the same conditions as above, as shown bycurve C in Figure 7. With a high resistance connected in series, KvbecomesKi divided by S and an error curve A' as illustrated in Figure 7results. With a smaller resistance in series the curves between A and Cresult. It is evident, therefore, that the innuence of frequency becomesa minimum for a certain value of S. In this case the error follows curveD. By utilizing suitable values of r, C. L, and R, such a small optimumvalue of S can be obtained, that the voltage measuring instrument mayalso be used as an ammeter arranged in shunt without the voltage dropacross the shunt resistance becoming inadmissibly high. The numbersappearing at the ends of the curves in Figure 7 represent therelationships for the various curves between the series resistance S andthe impedance of the combined current responsive unit at 50 cycles persecond.

I have herein shown and particularly described certain embodiments of myinvention and certain methods of operation embraced therein for thepurpose of explaining its principle and showing its application but itwill be obvious to those skilled in the art that many modifications andvariations are possible and I aim, therefore, to cover all suchmodifications and variations as fall within the scope of my inventionwhich is defined in the appended claims.

. What I claim as new and desire to secure by Letters Patent of theUnited States, is:

1. A frequency compensated voltmeter for alternating current circuitscomprising a current responsive unit and a resistor in series therewith,having a resistance relatively great in comparison with impedance of thecurrent responsive unit, said current responsive unit comprising arectifier bridge having an inductive current re- 4'Coll sponsiveinstrument element connected in a cross circuit thereof and a.capacitative impedance connected in parallel with the'rectier bridge.

2. A voltmeter for alternating current circuits comprising a currentresponsive unit and a resistor connected in series therewith, saidcurrent responsive unit comprising a rectier bridge having a crosscircuit in which an inductive current responsive instrument element isconnected and having a capacitative impedance connected in parallel withthe rectier bridge, the resistance of said series resistor being sorelated to the impedance of the elements of the current responsive Vunitas to compensate for unequalvariations 1n impedance of thev componentparts of the unit with variations in frequency.

3.-A frequency compensated voltmeter for alternating current circuitscomprising a current responsive unit and a resistor in series therewith,said current responsive unit comprising an inductive rectier bridgehaving a. current responsive instrument element connected in a cross4circuit thereof and a capacitative impedance CERTIFIGATE OF CORRECTION.Patent No. 2,213,086. Y Amst 27,119Lpo.

`mns F. GRAVE.

It is-hereby certified that error appearsl in'- the printedspecification ofthe above numbered patent requiring correotion esfollows: Page 5, second column, lines and 7, claim 5, for the words 'aninductive rectifier bridge having a" read --a rectifier bridgehayingvaninductive--g. and theft the said Letters Patent should be readwith this, correction therein1 that the same may conform to the recordof the case the Potent .Offiee Signed and sealed this 15th day ofQctOber', A. D. -1911.0.

Henry 'Van Arsdale, (Seel) Acting Commissioner of Patents.

